Statement: Declarative
sentence (grammatically well-formed series of words), that
represents something (its meaning)
that is true or not.

Non-declarative
sentences are e.g. questions or exclamations or exhortions, and are not
true or not. The reason for writing "true
or not" is that while "true or
false" is adequate for many purposes, it is
too narrow for some, such as when one considers contingent
statements about the future - such as "tomorrow it will rain" - which
may be plausibly claimed to be neither true nor false until the future
arrives, and one can find out.

The terms "statement" and "proposition"
tend to be confused, but don't need to come to the same. Indeed, one
often convenient definition of the latter
term is that a proposition is
the meaning of a statement - so that the same proposition may be stated
by different statements, either in the same language, e.g. with
synonyms, or in other languages.

Also, it should be noted that not all
well-formed declarative sentences in a natural language are statements
in the given sense. Chomsky has as an example "Colourless green ideas
sleep furiously", which is grammatical, but not precisely meaningful in
English, as the meanings of the terms it contains are commonly
understood.